In recent years, various optical disks such as CDs (compact disks) and DVDs (digital versatile disks) have been practically used as high-density information recording media. Recording and reproducing information to and from an optical disk are carried out with an optical disk unit having an optical system that emits a laser beam and forms a small condensed beam spot on an information recording layer of the optical disk.
More precisely, an optical disk 11 shown in FIG. 1 has an information recording layer 12. The information recording layer 12 is covered with a transparent substrate 13. In the information recording layer 12, information is written with record pits that are of the order of micrometers and are arranged in concentric circles or spirals. The transparent substrate 13 is made of transparent resin such as polycarbonate to protect the information recording layer 12 and provide the optical disk 11 with mechanical strength. An optical disk unit has a light guiding system to guide a recording or reproducing laser beam to the optical disk 11. From the laser beam, an objective lens (condenser lens) 14 in a condensing optical system arranged in the last stage of the optical disk unit forms a condensed beam spot 15 on the surface of the information recording layer 12.
The optical disk 11 shown in FIG. 1 is a two-sided recording optical disk with each face of the information recording layer 12 is covered with the transparent substrate 13. If it is a one-sided recording optical disk, the transparent substrate may be formed only over a recording side.
In recent years, optical disks are increasingly provided with large capacities and high densities. To catch up the tendency, the numerical aperture of the objective lens 14 for forming the condensed beam spot 15 is increased. The numerical aperture of an objective lens used for conventional CDs is 0.45 and that for high-density recordable DVDs is 0.6. Objective lenses having a numerical aperture of 0.8 or greater are expected.
In these circumstances, accuracy of the thickness t of the transparent substrate 13 on the optical disk 11 is very important when recording and reproducing information to and from the optical disk. The transparent substrate 13 has a refractive index. If the thickness t of the transparent substrate 13 is out of specification values when the objective lens 14 forms the condensed beam spot 15 on the optical disk, the condensed beam spot 15 will have a spherical aberration. As the numerical aperture of the objective lens 14 increases, the spherical aberration increases to heighten the possibility of causing a recording or reproducing error. If the objective lens has a numerical aperture of, for example, 0.85, an allowance for the thickness t of the transparent substrate 13 of the standard optical disk 11 is said to be several micrometers or smaller. Securing this accuracy is difficult even with the current technology of optical disk manufacturing.
FIGS. 2A and 2B show changes in light intensity distribution around an optical axis of the condensed beam spot 15 with respect to focus offsets. A dot-and-dash line indicates the optical axis of a laser beam. In FIG. 2A, the thickness t of the transparent substrate 13 is within an allowance, and in FIG. 2B, the thickness t of the transparent substrate 13 is out of the allowance. In FIG. 2A, the thickness t of the transparent substrate 13 is within the allowance, and the light intensity distribution of the condensed beam spot 15 shows that the diameter of the beam substantially symmetrically changes around a focal point.
In FIG. 2B, the thickness t of the transparent substrate 13 is out of the allowance, and there is a spherical aberration that causes concentric wavefront variations around the optical axis. As a result, the light intensity distribution of the condensed beam spot 15 shows asymmetrical changes around the focal point. The light intensity distribution shows irregular variations in beam diameter and produces side lobes that increase in proportion to focus offsets.
If the thickness t of the transparent substrate 13 is out of an allowance, the optical disk unit must correct a spherical aberration of the condensed beam spot 15 with some technique so that the condensed beam spot 15 may properly conduct recording and reproducing. There is a conventional method for correcting a spherical aberration of the condensed beam spot 15 caused when the thickness t of the transparent substrate 13 of the optical disk 11 is out of specification values. The method is disclosed in Japanese Unexamined Patent Application Publication No. 2002-150569 (hereinafter referred to as the Patent Document 1).
The spherical aberration correcting method disclosed in the Patent Document 1 will be explained. In a lead-in area (not shown) of the optical disk 11, the method forms a special pattern. In the special pattern, two kinds of pit strings 107 and 108 having different periods are alternately arranged as shown in FIG. 3A. According to the example of FIG. 3A, the period of the pit string 107 is longer than that of the pit string 108. A reproduced signal from the pit strings 107 and 108 has a signal waveform shown in FIG. 3B. A reproduced portion corresponding to the pit string 107 has a larger amplitude, and a reproduced portion corresponding to the pit string 108 has a smaller amplitude.
According to the Patent Document 1, the focal point of the condensed beam spot 15 is successively changed, to find characteristics between focus offsets and amplitudes as shown in FIGS. 4A and 4B, and according to the characteristics, the related art corrects a spherical aberration.
The characteristics of FIG. 4A are obtained when the thickness t of the transparent substrate 13 is within an allowance, and those of FIG. 4B are obtained when the thickness t of the transparent substrate 13 is out of the allowance. In FIGS. 4A and 4B, a continuous line represents changes in the amplitude of a reproduced signal from the long-period pit string 107, and a dotted line represents changes in the amplitude of a reproduced signal from the short-period pit string 108.
In FIG. 4A, there is substantially no spherical aberration. In this case, the amplitudes of the reproduced signals obtained from the condensed beam spot 15 are substantially symmetrical before and after (left and right in the figure) a focal point. In FIG. 4B, there is a spherical aberration. In this case, the amplitudes of the reproduced signals are asymmetrical before and after a focal point.
In FIG. 4B, fo1 and fo2 are focus offsets that provide maximum amplitudes for the reproduced signals from the long-period pit string 107 indicated with a continuous line and short-period pit string 108 indicated with a dotted line, respectively. The focus offsets fo1 and fo2 correspond to a spherical aberration caused by an error of the thickness t of the transparent substrate 13 exceeding the allowance. The signs of the focus offsets fo1 and fo2 that provide the maximum amplitudes of reproduced signals will invert depending on whether the thickness t of the transparent substrate 13 is thicker or thinner than the allowance. Accordingly, it is possible to determine not only the degree of the spherical aberration but also the direction thereof. The aberration shown in FIG. 4B is referred to as an aberration of positive direction.
Controlling an optical system in such a way as to eliminate (minimize) the focus offsets fo1 and fo2 will correct the spherical aberration and optimize the condensed beam spot 15. This is the spherical aberration correcting method disclosed in the Patent Document 1.